Embodiments described herein relate to a catalyst for cold-start engine emissions which contains oxygen storage materials to provide a source of oxygen for hydrocarbon combustion at low temperatures. More particularly, embodiments described herein relate to a hydrocarbon trap including the catalyst.
In recent years, considerable efforts have been made to reduce the level of carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxide (NOx) emissions from vehicle engines. Conventional exhaust treatment systems typically include a three-way catalyst (TWC) to reduce these pollutants and prevent the exit of unburnt or partially burnt hydrocarbon emissions from the vehicle exhaust. However, during initial starting of the engine, the three-way catalysts are not sufficiently hot to become catalytically active, i.e., they have not reached their “light-off” temperature. As a result, pollutants may pass through the engine exhaust system without being treated.
Hydrocarbon traps have been developed for reducing emissions during cold-starting by trapping/adsorbing hydrocarbon (HC) emissions at low temperatures and releasing/desorbing them from the trap at sufficiently elevated temperatures (i.e., at or above the light-off temperature) for treatment by a three-way catalyst for conversion to CO2 and water. The three-way catalyst may be located downstream from the trap or it may be combined with the trap adsorbent material, for example, by providing a TWC washcoat layer over the HC adsorbent material (referred to as a catalyzed hydrocarbon trap) where both the TWC and HC adsorbent material are deposited on a monolithic carrier. Typically, a catalyzed hydrocarbon trap comprises a three-way catalyst material with an adsorbent material such as zeolite that traps the hydrocarbons during cold-starting.
In operation, the exhaust flow is then routed across the hydrocarbon trap such that HC species adsorb on the trap. Upon desorption from the trap, the hydrocarbons contact the three-way catalyst for conversion to CO2 and H2O as the trap and catalyst heat up.
One of the greatest challenges with the use of such hydrocarbon traps in an exhaust system is to retain the adsorbed hydrocarbons until the three-way catalyst is hot enough to efficiently convert the HC when desorbed from the trap. This is typically facilitated by combining the HC trapping material (such as zeolite) and catalyst into a single body such as a monolith honeycomb. However, it is still possible for some HC species to desorb from the trap below 200° C., which is below the light-off temperature of a typical three-way catalyst formulation. In addition, conventional exhaust gas systems typically include an upstream TWC catalyst which is close-coupled to the exhaust manifold, and thus warms up more quickly than the downstream HC trap. Hence, oxygen is significantly depleted from the exhaust stream by oxidation reactions occurring over the close-coupled TWC catalyst, so there is often insufficient oxygen left to react with the released HC from the trap to form CO2 and H2O. This can cause HC to slip out of the trap untreated, resulting in low HC conversion.
These problems increase with the aging of the trap. For example, high temperature aging during vehicle operation causes stored hydrocarbons to desorb at lower temperatures and requires higher temperatures to achieve oxidation of the released hydrocarbons. While lower operating temperatures have been achieved by placing the a hydrocarbon trap in the underbody converter assembly of the vehicle, exhaust gas oxygen required to enable conversion of trapped HC to CO2 and H2O is limited in this location.
Further, typical temperatures of gasoline exhaust gases upstream of close-coupled catalysts are generally over 800° C. Thus, the materials used as hydrocarbon absorbents must have high temperature stability. While gamma alumina and zeolites have been used in conventional hydrocarbon traps, they tend to lose adsorption capability at such high temperatures.
Accordingly, it would be desirable to provide an improved catalyst and source of oxygen which achieves oxidation of hydrocarbons at low temperatures, and to provide a catalyst/hydrocarbon trap which maintains good hydrocarbon adsorption capability and catalyst conversion over the useful life of the vehicle.